Bloomberg is making the endorsement because of concerns about Republican presidential nominee Donald Trump

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Former New York City Mayor Michael Bloomberg speaks at the Sierra Club April 8, 2015 in Washington, DC.

Michael Bloomberg is expected to endorse Hillary Clinton during his speech at the Democratic National Convention, an adviser for the former New York City mayor told NBC News.

Bloomberg is making the endorsement because of concerns about Republican presidential nominee Donald Trump. He also sees himself as a person with authority who can debunk Trump's claims about business and the economy, the former aide said.

Bloomberg has not been a member of the Democratic Party since 2000, according to The New York Times. He was elected the mayor of New York City as a Republican and later became an independent.

Bloomberg announced in March he wouldn't be running for president, saying: "When I look at the data, it's clear to me that if I entered the race, I could not win. I believe I could win a number of diverse states — but not enough to win the 270 Electoral College votes necessary to win the presidency.”

Trump Booed Leaving New York Times

President Elect Donald Trump is booed as he walks through the lobby of The New York Times Building after a 75-minute meeting with Times journalists. The lobby of the Times building is open to the public, and a large crowd had gathered by the time he departed.

(Published Tuesday, Nov. 22, 2016)

He also said at the time Trump "has run the most divisive and demagogic presidential campaign I can remember, preying on people's prejudices and fears," citing Trump's proposal to ban most Muslims from entering the United States and deporting those here illegally.

In May, Bloomberg told CNBC, "Trump marches to his own drummer it would seem, and so far, that drummer has been playing the right tune to get him this far."